1. Field of the Disclosure
This disclosure relates to mesoporous metal oxides and processes for making mesoporous metal oxides, in particular, the synthesis of thermally stable mesoporous metal oxides with controllable nano-sized wall crystallinity and mesoporosity. This disclosure also relates to a method of tuning structural properties of mesoporous metal oxides, and a method of controlling nano-sized wall crystallinity and mesoporosity in mesoporous metal oxides.
2. Discussion of the Background Art
Porous transition metal oxides consist of micropores (<2 nm), mesopores (2-50 nm), micropores (>50 nm) and sometimes combinations of these. Considerable interest in the control of pore sizes and pore size distributions of such materials has been a focus for quite some time. The control of particle size in particular in the nanometer regime in the synthesis of nano-size metal oxides is also currently being pursued. Nano-size materials can have markedly different properties than similar compositions that are bulk size (μm and above). Control of morphologies of porous transition metal oxides such as hollow spheres, rods, helices, spirals, and many other shapes has been a major focus of researchers over at least the last 10 years.
Such control comes from specific synthetic methods such as use of templates, structure directors, surfactants, core shell, self assembly, epitaxial growth, size reduction, capping agents, sol gel, and other methods. Morphologies can be controlled by compositions including dopants. The conditions during syntheses such as use of heat, light, pH, point of zero charge, stirring, high pressure, and others are also important.
Mesoporous materials with varied pore sizes and pore size distributions can be obtained for some systems such as silicon and titanium based oxide materials. However, control of pore size distributions to make single size pores and to systematically control such pore sizes and uniformity is difficult, especially with transition metal oxide systems. Control of the structure of the material is also an issue. Many systems have both micropores and mesopores and pore interconnectivity is of interest with these materials. Enhanced mass transport for catalytic reactions might be realized by fine-tuning the porosity of such systems. Incorporation of biomolecules larger than the micropore regime also might be done using well ordered crystalline mesoporous materials.
Most studies of mesoporous transition metal oxide (MTMO) materials have focused on groups I-IV including Y, Ti, Hf, Zr, V, Nb, Ta, Cr, Mo, and W. These have low angle X-ray diffraction peaks indicative of mesostructural ordering and Type IV isotherms. These syntheses have focused on use of water or water plus a base or urea with various amine and carboxyl containing surfactants (S). Other syntheses have been conducted in an alcohol (mainly ethanol) and in the presence of either an acid (mainly hydrochloric acid) or a base. There are either strong Coulombic interactions (S+, I−; S−I+; S+X−I+; S−X+I−) or strong ligand metal interactions (I:S<2, very thin walls), and such systems have limited thermal stability and amorphous walls, where I=inorganic species, and X is a mediator. Such syntheses are open to air and various aging times and environmental conditions can influence the porosity of these materials.
Water content is a critical parameter with the synthesis of porous transition metal oxides. Water competes with ethoxy and other alkoxy groups for coordination to the metal and also significantly affects hydrolysis and condensation rates. Since most syntheses are open to the air the water content is very difficult to control. On the other hand, water is essential for reaction. When the number of water molecules per metal atom (H) is >1 then phase separation and nonporous oxides result. When H is <1, ordered mesoporous materials are formed when the metal has empty t2g orbitals. These materials obtain water from the environment during synthesis. When H is <<1, strong surfactant/transition metal interactions occur with weak surfactant surfactant interactions and there is no reaction.
Thermodynamic interactions in such syntheses and factors influencing each term are given in Table 1 below. Table 1 sets forth thermodynamic parameters of surfactant (S) transition metal (M) mesopore syntheses.
TABLE 1DGm = DGorg + DGI + DGinter + DGsol [1]S-S InteractionHigh LewisStrong S-MUnknown anddeterminesacidityinteraction atunpredictablemesostructureUnsaturatedinterfaceformedCoordination(Coulombic,(Lamellar,H (HydrolysisCovalent bonding,Hexagonal,Ratio H<<1),HydrogenCubic)Condensationbonding)hinderingmolecules(carboxyl, amine,ethylene glycol)
In Equation 1 above, DGm is the formation energy of the mesostructured material; DGorg is the surfactant-surfactant interaction; DG1 is the metal-metal interaction; DGinter is the surfactant-metal interaction; and DGsol is the solvent interaction. It would be desirable to develop a process that minimizes the last 2 terms, DGinter and DGsol, in order to make well ordered MTMO materials. The absence of totally empty d orbitals restricts the strong interaction between surfactant and metal (ligand to metal charge transfer) which is generally accepted as essential for the formation of ordered materials. Filled t2g orbitals such as in systems containing Mn, Fe, Co, and others are difficult to make with the above methods since charge transfer reactions do not occur.
The present disclosure provides many advantages over the prior art, which shall become apparent as described below.